Polyester, particularly polyethylene terephthalate made from an aromatic dicarboxylic acid and glycol, being industrially produced in the present time are widely used in fibers, films, moulding materials and the like due to remarkable mechanical, physical and chemical properties thereof. Polyethylene terephthalate is produced industrially by the process which comprises heating bis(.alpha.-hydroxyethyl) terephthalate and/or a low molecular weight condensate thereof(referred to the esterified products hereinafter) together with a polycondensation catalyst at a temperature of 260.degree. to 300.degree. under reduced pressure to effect polycondensation. Bis(.beta.-hydroxyethyl) terephthalate and/or low molecular weight condensate can be obtained by the direct esterification which consists of heating terephthalic acid and ethylene glycol at a temperature of 200.degree. to 280.degree. C. under atmospheric pressure or increased pressure, or the ester-exchange reaction which consists of heating dimethyl terephthalate and ethylene glycol at a temperature of 140.degree. to 240.degree. C. in the presence of a catalyst (referred to TPA method and DMT method hereinafter, respectively).
In general, a polycondensation catalyst has been used to develop easily a polycondesation when producing polyester in a commercial scale. The polycondensation catalysts include metal compounds such as antimony, titanium, germanium, tin, zinc, manganese or lead compound, and they effect significantly polycondensation rate and a quality of the obtained polyester, since a polycondensation is carried out at a high temperature for a long time in the presence of metal compound catalysts. The polyester having a high polymerization degree is obtained easily by these catalyst, whereas the unfavorable side effects are accompanied by the catalyst result with the problem to color the obtained polymer yellow or gray, or to deteriorate its physical properties as lowering a melting point and strength when increasing the content of diethylene glycol(DEC) and terminal carboxyl group(COOH) over an appropriate level.
Therefore, a process for the preparation of polyester having excellent physical properties and a good color tone for a short period of reaction time is very essential in the productivity and the quality of products.
With regard to the above facts, the most widely used polycondensation catalysts for the preparation of polyester in the industry include antimony compounds and germanium compounds, especially antimony trioxide and germanium dioxide.
Antimony trioxide is effective in cost, its catalytic activity in a polycondensation is relatively high and its side effects are not serious, while since it hardly dissolves in ethylene glycol or the reaction mixture and tends to precipitate during the reaction, the finally obtained polyester may have undesirable orange-green or gray tone, or appear reduced clarity. In case of germanium dioxide, the side effects may be reduced a little, whereas a polymerization time may be lengthened due to the lower catalytic activity of germanium dioxide than that of antimony trioxide.
Side effects and coloring should be careful since they have a bad influence on the quality of the resulting polyester. To increase an amount of catalyst or elevate a reaction temperature in order to shorten a polycondensation time and to enlarge the productivity, the problems such as side effects and coloring become remarkable.
Many methods to improve the known problems above have been suggested for reducing a reaction time by a catalyst and for producing polyester having excellent physical properties and a good color. However, the proper method is hardly found.
For increasing a polycondensation rate, a method using a reaction of silicone compound with titanium compound is described in U.S. Pat. No. 3,927,052, a method using germanium compound together with titanium compound in UK patent No.949,085, a method using a mixture of antimony trioxide, cobalt compounds and phosphine compounds dissolved in ethylene glycol in JP patent Laid-Open No. 51295/1978 and a method using a reaction product of titanium compounds and an organic acid in U.S. Pat. No. 4, 131,601. However, in accordance with these methods the color of the resulting polymer appears pale-yellow, or many problems happen to the physical properties thereof due to an increase of the content of diethylene glycol and terminal carboxyl group.
In order to improve a color of the polymer, JP patent publication No. 28119/1972 discloses a method using cobalt compounds as a color-improving agent, particularly using it together with titanium compound, JP patent Laid-Open No. 73827/1990 does a method using cobalt compound with a mixed catalyst of antimony and tin, and JP patent Laid-Open No. 117216/1983 does a method using cobalt compounds and alkali metal compounds with antimony compounds. However, the above methods are not able to improve both a color and clarity and physical properties of the polymers at a same time.
As mentioned above, although a number of studies have been done to resolve the problems of the prior art, they do not satisfy the reduction of the polycondensation time, the inhibition from coloring and the improvement of the decrease of physical properties, simultaneously.